1. Field of the Invention
This invention relates to an improvement in a method for forming a thin film comprising a combination of ion beam deposition and vacuum evaporation, and more specifically, to a method and an apparatus for producing a thin film of high quality, for example a thin high-quality film of boron nitride (to be referred to as BN) of high hardness.
2. Description of the Prior Art
The present inventors previously proposed a novel method for forming a thin film comprising a combination of ion irradiation and vacuum deposition in Japanese Laid-Open Patent Publication No. 2022/1983. This method is directed to the formation of a thin film of a metal compound on the surface of a substrate by ion beam deposition, in which accelerated ions are irradiated onto the substrate, and a vapor of the metal compound is irradiated on the substrate simultaneously or alternately with the irradiation of the ions. According to this method, the evaporant to be deposited is chemically combined with the ions by utilizing the activated energy or kinetic energy of the accelerated ions and a new material is formed on the substrate.
FIG. 2 of the accompanying drawings is a schematic view showing a typical apparatus for carrying out this method of thin film formation. A gas to be ionized, for example nitrogen, is introduced into an ion source 2 via a leak valve 1 and ionized there. The ions are then accelerated by an accelerator 3 to impart a predetermined ion accelerating energy. The ions are then introduced into an analyzer magnet 4 where only the required ion species are magnetically selected and supplied to a reaction chamber 5.
The reaction chamber 5 is maintained under a high vacuum of 10.sup.-4 torr or less by a vacuum pump 6 (for example, a turbo molecular pump). A substrate 7 is fixed to a substrate holder 8 and the selected ion species are irradiated on the substrate. In order to irradiate the ion species uniformly on the substrate 7, it is desirable to pass the ion species through a focusing lens 9.
An evaporation device 10 is disposed below the substrate 7. This device is heated by a suitable method, for example by electron beam heating or laser beam heating. The evaporation device 10 includes an evaporation source containing B, for example. The amount of the evaporation source containing B to be deposited and the deposition speed can be measured by a vibratory film thickness tester 11 including a quartz plate, for example, which is disposed side by side with the holder 8.
The number of atoms of the ion species, i.e. the ionic current, can be accurately measured by an integrating ammeter 13 having a secondary electron repelling electrode 12 annexed to it.
A voltage-adjustable bias power supply 14 is connected between the substrate 7 and the secondary electron repelling electrode 12 so that a negative bias voltage is applied to the substrate 7.
In this device, the substrate 7 is set at a predetermined position, and the inside of the reaction chamber 5 is maintained at a predetermined degree of vacuum. By operating the evaporation device 10, the evaporation source containing boron is evaporated and deposited in a predetermined amount on the substrate 7. Furthermore, a predetermined ion species is irradiated on it with a predetermined ion acceleration energy. When at the same time, a predetermined negative bias voltage is applied to the substrate, a thin film of BN having a high hardness and composed mainly of cubic BN (CBN) and hexagonal closed packing BN (WBN) is formed on the surface of the substrate 7.
Since the composition of the thin film to be formed is determined by prescribing the ratio between the ions to be irradiated and the deposited atoms, thin films of different compositions, for example a BN film, can be easily produced by varying this ratio.
In the aforesaid method of thin film formation, the path of the ion atoms to be irradiated toward the substrate 7 from the ion source 2 and the path of the evaporating atoms to be irradiated toward the substrate 7 from the metal vapor evaporating device do not exist in the same direction. Hence, according to the apparatus shown in FIG. 2, the evaporating atoms are projected at an inclined angle toward the substrate 7, and owing to minute raised and depressed portions on the surface of the substrate 7 or a film-forming surface, shaded parts occur microscopically which remain non-irradiated with the evaporated atoms. Uniform surface irradiation is, therefore, impossible.
Furthermore, although it would be easy for the ion atoms to attain a highly excited level on the substrate 7, the evaporating atoms are not activated. If, therefore, the evaporating atoms are deposited after they are caused to gain an activated energy state, the reaction and combination with the ion atoms can favorably be facilitated.
Furthermore, in the thin-film forming apparatus based on this conventional method, an ion-forming gas is introduced into a plasma generating zone of an ion source to generate ions. During this time, all the electrons generated simultaneously flow to the ground through the wall surface of the ion source, and consequently, the high energy of the electrons is wastefully discharged.